Why does a thin layer of fog form over lakes, ponds and rivers in the early morning hours during the fall?
Bodies of water, such as lakes, ponds, and rivers, are much slower to cool down than land areas are. During clear fall nights, the warmth of the land escapes into space. As the air over the land cools, it will drift over the warmer lake. A thin layer of air above the lake is warmed by the lake water. Water evaporates from the lake’s surface into this thin layer. The thin, warm, moist layer of air over the lake then mixes with the cooler air from the land. As it cools, condensation occurs and a fog forms. It looks like steam rising off the water, hence the name “steam fog.” In the spring, the ponds are usually colder than the surrounding land. Just as they are slow to cool, they are also slow to warm. (Barbara McNaught Watson)